Diesel engines have higher efficiency than gasoline engines due to the increased compression ratio of the diesel combustion process and the higher energy density of diesel fuel. As a result, a diesel engine provides improved gas mileage over an equivalently sized gasoline engine.
The diesel combustion cycle produces soot (diesel particulates) that is typically filtered from the exhaust gases. A diesel particulate filter (DPF) is usually disposed along the exhaust stream to filter the soot from the exhaust. In one type of DPF, the filter is a honeycomb filter. Over time, the soot builds up in the DPF and the DPF must periodically be regenerated to remove the entrapped soot. One method of regeneration is to burn the soot within the DPF to enable the DPF to continue its filtering function.
The temperature of diesel exhaust during normal operation, for example 150-250° C., is considerably lower than what is required to thermally regenerate a saturated DPF. To initiate a self-propagating particulate combustion event, temperatures in the approximate range of 550-850° C. must be achieved. Consequently, some method for raising the DPF temperature beyond what is encountered in the exhaust must be employed during the regeneration cycle.
Some methods known in the art for raising the DPF temperature include indirectly raising the temperature of the DPF by increasing the temperature of the exhaust gas, for example, through catalytic oxidation of excess fuel or through electrical heating of an element upstream of the DPF. However, in either of these two approaches, not all of the heat transferred to the exhaust gas is transferred to the DPF. Much of the exhaust gas passes through the DPF with incomplete heat transfer, creating a large inefficiency. Moreover, for the fuel burner, the inefficiency is compounded by the creation of additional particulate and hydrocarbon emissions, a lower exhaust oxygen concentration, and shorter lifetimes for the DPF due to cracking from thermal gradients.
Another method known in the art for raising the temperature of the DPF to the particulate combustion temperature is through microwave heating of a DPF made from a suitably absorbing ceramic material. To achieve this, either the entire DPF or at least selected regions of the DPF must be made of a material that is able to absorb microwave energy at the frequency of operation. For example, the entire DPF may be made from an expensive microwave absorbing ceramic material such as SiC, or a standard cordierite DPF may be selectively coated with an absorbing material. In both cases, parasitic absorption of microwave energy by the particulates effectively reduces the regeneration efficiency to an intolerable level.